Xerox Corp.'s multibillion-dollar commercial and residential project near Leesburg was approved by the Loudoun County Board of Supervisors yesterday in a unanimous vote to rezone the 2,267-acre site along the Potomac River.

Potomac Park, the proposed headquarters for 13 large corporations and expected site of 1,830 homes, will be the largest development of its kind in the Washington area when completed around 2000, Xerox attorney William G. Thomas said yesterday. Groundbreaking for the project, which will open in phases, is expected within the next several months, Xerox official Benjamin N. Knupp II said.

"I'm proud . . . to vote for probably the best thing that's come to Loudoun County since Dulles Airport," said Supervisor Andrew R. Bird III.

Potomac Park is one of several recently approved and proposed developments that are completing the transformation of eastern Loudoun from an expanse of dairy farms into a suburban community, a process that began more than 20 years ago with the opening of Dulles International Airport.

In addition to early suburban projects like Sterling Park and Countryside, Potomac Park will be neighbors with Ashburn Village, a 5,000-unit community approved by the Loudoun board earlier this month, and Ashburn Farms, a proposed 1,200-unit development expected to face the board next year.

The only potential snag to approval yesterday came from Leesburg Mayor Robert E. Sevila, who asked that the board delay approval until a dispute is resolved between Leesburg and Loudoun County over which jurisdiction will provide sewer service to one section of Potomac Park.

The board, rejecting Sevila's plea, voted to postpone deciding which of two apparently contradictory agreements the county should honor. A 1972 agreement established that the county would provide sewer service to the entire Xerox property.

Sevila, however, said that a 1984 accord between Loudoun and Leesburg required that the county and the town reach mutual agreement on which jurisdiction would provide sewer expansions in an area around Leesburg, including Potomac Park.

"We're not looking for a fight over this," Thomas said in an interview yesterday. "We're willing to go along with whatever is decided . . . . However, I didn't believe that a utility question should be part of a land-use decision, and . . . that's why we didn't want to wait."

Thomas said Potomac Park, which will include a golf course, and a resort hotel and conference center, will prove attractive to corporate tenants because of its green, open-spaced setting, and its strategic location about 10 miles north of Dulles International Airport. The Xerox International Training Center is already located at the site.

Loudoun officials, likewise, have been trumpeting the development since its announcement last April because of the enormous tax base the project will provide, and because of the willingness of Xerox officials to supply generous concessions to the county in exchange for zoning approvals.

Xerox offered more than $25 million in public improvements to Loudoun in exchange for yesterday's approval. Xerox will build a large road network inside Potomac Park and construct two highway interchanges connecting the project's roads with Rte. 7, a heavily traveled commuter road into the Virginia suburbs.

In addition, Xerox officials have said the project eventually could bring more than 30,000 jobs into the county.